Friday, July 23, 2010

My second take on the battery protector.The way it works now is: I added a simple monostable relay. The protection circuit itself is powered through that relay. Basically when connected to the battery, nothing happens. There's a push button switch between Battery + and the relay coil end designated to be the positive. When the button is pushed the relay switches on momentarily, the circuit becomes active and if the voltage is above threshold the FET continues to hold the relay on when the button is released. If not then there's nothing to keep the relay on so it just switches off.Since the FET is only driving the relay coil there's no heat dissipated there.Also no chance of oscillation: When the voltage dips below threshold the relay cuts out and that's that.Possible unnecessary and overkill, but whatever. It works.According to my cheapo DMM It consumes 50milliamps when holding the relay on, which translates to about a 0.6W average. I can live with that.

Friday, July 16, 2010

Done with them for now. Got a Tripath TA2020-based Chinese (Lepai) amp off eBay to use with these. Should get here in a couple of weeks. I chose this amp because: 1, The amplifier chip itself it worthy to be one of the 25 chips that shook the world according to IEEE Spectrum. 2, There is a thread on diyAudio detailing several all-around improvement mods for this particular amp. But most people seem to agree that it sound at least "pretty good" even without them (Though that certainly won't stop me from doing them). 3, It was cheap ($20 shipped)

Should be interesting to compare to the TDA2004 board I used for testing. Which I could describe in one word as: unspectacular.

Sunday, July 11, 2010

Housing made out of scrap (not to mention low quality, and pretty old) MDF from some trashy office furniture. Cut the boards out with a jigsaw, screwed them together, filled the gaps with wood putty. Gave it a run with a power sander for good measure.

Upholstered them with some white fabric. (I know it's a bad choice, but it's what I could get for free and I don't intend to spend much on this ) I haven't decided yet whether I'll paint it over black.

These are the speakers that will go in there. They're some leftover "5way" car speakers with a "ProAudio" brand.

Don't really have a schematic for this small project. I had an MC34161 (datasheet PDF link) glorified comparator voltage monitor IC in an old inverter PSU that I decided to re-use to protect a single SLA battery against over-discharge. Only one voltage monitor channel is used, and the chip is configured to be an overvoltage detector. The open collector output it pulled high with a random resistor and it's connected to the gate input of a random N channel FET. The MC34161 is calibrated with a precision trim-pot (not the one in the picture, I replaced that because the threshold would crawl 0.5V just by blowing on it :) ) to detect overvoltage at around 10.6V. At which point it'll switch the FET enabling the output terminals. If the voltage sinks below 10.6V it'll disconnect the battery from the output. (overvoltage condition disappears)

I actually haven't tested whether the chip's built-in hysteresis is enough to prevent oscillation yet (large load on battery -> battery discharges below 10.6V -> battery is disconnected -> battery voltage rebounds ? -> ? above hysteresis threshold -> repeat until something melts/breaks/explodes)This is obviously something to avoid. It's probably not a issue for me because it doesn't matter for the devices I'll be powering off this battery. (I'd just realize what's going on and power them off) but it's something to keep in mind in case anyone builds something similar based on this chip.

UPDATE: Ok, you probably shouldn't build this unless you use an extremely low RDS-on FET or something. I'll be modifying this to work as an undervoltage detector and to use a bistable relay when I have the time. This'll also have the advantage of solving the hysteresis issue (the relay will just switch off when the undervoltage detector triggers it). If I set up the chip as a dual detector I could use it to switch the relay back on when the voltage rises above say.. 13 volts. I'll either do that, or just add a "reset protection" button if I'm lazy. (I'll probably just do that)